Generally, subsea optical communication systems communicate over long distances by operating at constant optical power. The designed power of a data channel in the system is the total constant power (in dBm or milliwatts) divided by the number of data channels that can fit within the repeater bandwidth of the subsea communication line. When the system is not fully populated with data channels, such as during early installations or during upgrades where fewer higher capacity channels can replace many legacy channels, the power per channel of those fewer channels will be higher than the designed optimal power. In contrast, terrestrial communication lines generally use constant gain amps, that is, as channels are added power is added such that the power is the same for each channel.
Subsea communication systems were designed originally for a format running at higher power levels than needed or desired by current systems. Typically, in order to operate a desired number of data-carrying channels at a desired power level in subsea communication systems, idlers (also known as dummy lasers) create idler signals for transmission in non-data carrying channels to soak up the excess power, that is, to use the unwanted milliwatts. The idler channels do not carry data and are used as filler to maintain the desired power level of the data-carrying channels. If additional data-carrying channels are desired, the data channels can replace or lower the power of one or more idler channels.
Examples of current idlers include spectrally filtered amplified spontaneous emission (ASE) devices and single polarization continuous wave (CW) lasers. In current subsea communication systems, however, current idlers cause problems with the transmission of data in the channels. ASE and CW idlers cause degradation of quality of the data optical signals in the data channels. For example, since ASE idlers are noise based, the amplitude noise can impart a nonlinear penalty to the data channels through cross-phase modulation (XPM). Since current CW laser idlers are singly polarized sources, the singly polarized CW laser idlers can create linear and nonlinear penalties to the polarization multiplexed data channels phase modulated signals through polarization dynamic gain (PDG) known as “hole burning” and cross-polarization modulation (XPoIM) non-linearity issues.